For projection systems that use reflective liquid crystal display (LCD) imagers, a folded light path where the illuminating light beam and the projected image share the same physical space between a polarizing beam splitter (PBS) and an imager offers a compact design. Most reflective LCD imagers are polarization rotating, i.e., polarized light is either transmitted with its polarization state substantially unmodified for the darkest state or transmitted with its polarization state rotated to provide a desired gray scale. Thus, a polarized light beam is generally used as the input beam. Use of a PBS offers an attractive design because it can function to polarize the input beam and fold the light path.
A PBS is an optical component that splits incident light rays into a first (transmitted) polarization component and a second (reflected) polarization component. One common PBS is the MacNeille polarizer that discriminates between s and p-polarized light as described in U.S. Pat. No. 2,403,732 to MacNeille. In a MacNeille polarizer, the s-polarization is reflected and, over a narrow range of angles near the Brewster angle, the p-polarization is mostly transmitted. The p-component corresponds to light polarized in the plane of incidence. The s-component corresponds to light polarized perpendicular to the plane of incidence. The plane of incidence means a plane defined by a reflected light ray and a normal to the reflecting surface.
Some skilled in the art have devised other types of PBS. For example, U.S. Pat. No. 5,912,762 (Li et al.) discloses a thin film polarizing device that may be used in a PBS. The device has first and second light transmissive substrates in the form of prisms and a plurality of thin film layers disposed between the prisms. The thin film layers comprise high refractive index layers and low refractive index layers, the high refractive index layers having one or more different refractive indices and the low refractive index layers having one or more different refractive indices. The light transmissive substrates have a refractive index greater than the refractive index of each of the low refractive index layers. The prisms are shaped so as to allow incident light to strike upon the thin film layers at a plurality of angles greater than or equal to the critical angle (i.e., the angle that generates total internal reflection conditions) for the highest refractive index of the low refractive index layers. Like the MacNeille polarizer, the polarizer in U.S. Pat. No. 5,912,762 discriminates between s and p-polarized light, although in the latter, s-polarized light is transmitted and p-polarized light is reflected.
As another example, WO 00/70386, in FIG. 1, discloses a Cartesian PBS element 50 that includes a multi-layer birefringent film 52 encased in a glass cube 54, and oriented so as to reflect light incident with x-polarization (i.e., approximately s-polarization). See page 11, lines 9 to 11. The notation in WO 00/70386 publication is different in that y-polarization is said to be approximate to s-polarization. For incident rays of light in a large cone angle, the Cartesian PBS has been demonstrated to provide a higher contrast than a PBS that discriminates only on the basis of s-polarization vs. p-polarization.
The technology discussed thus far, although disclosing useful PBS using multi-layer films, may not be well suited for use in a projection system. In such a system, the PBS typically experiences high intensity of light from a wide range of wavelengths possibly for long periods of time. Although the inorganic based multi-layer films of U.S. Pat. No. 2,403,731 and U.S. Pat. No. 5,912,762 may be stable to high intensity blue light, they have deficiencies in angular performance needed in low f-number systems. What is needed to advance the art is a multi-layer film based PBS that has the durability to withstand the light source and simultaneously to provide contrast for incident light in large cone angles so that the resulting image of a projection system, when viewed by an observer, appears bright, sharp, distinct, and possesses crisp colors.